For example, in the alphabet root typeset (see alphabet-main(2)), a string is represented by the letter ``s'', held at the Limbo level as a ref Value.S.

Each alphabet module has a type signature which describes its return type and the number and type of any flags or arguments that it allows. Inside an alphabet typeset, this signature is represented as a simple string where the first character (always present) indicates the return type of the module. Subsequent characters up until the first minus (``-'') sign (or the end of the string) indicate the module's required argument types. If the last character is an asterisk (*), it allows an unlimited repetition of the preceding argument type.

These may be followed by any number of options, each indicated with a minus character, followed by the option character and then the type characters of any arguments it requires.

For instance, the following Alphabet declaration:

	/mount [-abc] [-x /string] /wfd /string -> /status

An external alphabet typeset is conventionally represented by two header files defining the interface to the typeset, and two modules giving its implementation. Suppose we are to create a new typeset, say /foo . We would create the following files:

/module/alphabet/foo.m

Foo.m declares the interface used by all modules within the typeset. The existing typeset interface files (for instance alphabet/grid.m, documented in alphabet-grid(2)) provide examples of this kind of interface.

/appl/alphabet/footypes.b

This module translates between internal values (each held as a Value as declared in foo.m) and external values (each held as a Value as declared in the parent typeset, in this case by the alphabet module itself). Since Limbo does not provide a way of holding an arbitrary type directly, internal values are instead stored in a table by a local proxy (see below), and referred to externally by their index there.

/appl/alphabet/foo.b

Foo.b provides the basic type-manipulation primitives needed by the typeset, for instance the translation from type character to type name. It is also a convenient place to implement helper functions to make using the typeset easier. For instance, it is conventional for a typeset's Value adt to contain one eponymously named member function for each type character, making sure that the Value is actually of that type and returning the widened type, or raising an exception otherwise. For instance, in the root typeset, v.s() returns the type ref Value.S, or raises an error if v is not of that type.

/module/alphabet/footypes.m

Footypes.m provides an interface to the typeset proxy module, footypes.b, that allows direct access to values in the foo typeset, while still allowing manipulation of those values by an alphabet instance.

The proxy module, footypes.b, must define at least one function, proxy, which returns a channel through which all operations on the typeset take place. The Proxy module (see alphabet-proxy(2)) provides a generic implementation of such a translator; if footypes.b uses this, it needs only define the mapping between values in its parent typeset and its own values.

types()

Types is always the first function in a module to be called. It should do nothing but return the type signature string of the module.

init()

Init is called to allow the module to initialise its global state. It is called once only. It is permissible for this function to raise a ``fail:'' exception on failure. The text following the ``fail:'' prefix should describe the reason why.

run(errorc, r, opts, args)

Run runs an actual instance of the module. It must be re-entrant. The signature of the run function varies from typeset to typeset, but usually includes the above arguments. Args holds a list of the arguments passed to the module; opts holds all the flags that have been specified. Each flag is represented with a tuple, say: (c, optargs), where c gives the option character, and optargs is a list holding any arguments the flag requires. The arguments and options passed to the module are guaranteed to conform with the type signature returned from types. Note that each flag may be passed multiple times to the module.

If the run succeeds, it should return the resulting value. If the module returns a value that was passed in, and it contains a reference-count, the count should be incremented before returning, If the module succeeds, it is responsible for the disposal of any arguments and option arguments that it has been given. Appropriate disposal depends on the type of the argument, but v.free(0) is always sufficient to dispose of value v.

If the run fails, it should return nil; its arguments will automatically be freed in this case.

While processing the run request, the module should send error and debugging diagnostics to the errorc channel (it should take care never to send an empty string). If it spawns any new processes, it can use the Report, r, (see alphabet-reports(2)) to create new diagnostic channels for these processes. When such an diagnostic channel is no longer in use, the module should send an empty string on it. It should take care that Report.start is called before run returns.